Many 3D apps, including the ones I use has a built in depth of field for the rendering camera as a filter. Basically, you choose the object you want the camera to focus clearly on, then set the blur difference. The camera will automatically determine the level of blur from those objects closest to the focused object, as from the objects much more separated from the focus.

You can apply the Depth of Field filter to animated 3D scenes as well. Combining depth of field with motion blur can really make a scene both realistic and more difficult to discern due fighting blurs.

GP